Single return path orthogonally-arranged optical focus and tracking sensor system

ABSTRACT

An apparatus and method for providing tracking and focus error signals for controlling an application of a radiation beam to a data track of an optical storage medium. A data signal, indicative of data stored on the medium, may also be provided. One apparatus in accordance with the present invention includes a multi-element beam separator, such as a quad prism, having first, second, third and fourth separator elements for separating a return beam reflected and diffracted from the data track into first, second, third and fourth portions, respectively. In other embodiments, more or less than four separator elements and return beam portions could be used. A focusing lens converges the portions of the return beam onto a detector array. The first and second separator elements, arranged on opposite sides of a plane substantially parallel to a reference plane defined by an optical axis of the radiation beam and a tangent to the data track, separate the return beam along at least one plane substantially parallel to the reference plane, and the resulting first and second portions are used to generate a tracking error signal. The third and fourth separator elements, arranged adjacent to and on opposite sides of the first and second separator elements, separate the return beam along planes substantially perpendicular to the reference plane, and the resulting third and fourth portions are used to generate a focus error signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention is related to the following U.S. PatentApplications: "Apparatus and Method for a Dual Half-Aperture FocusSensor System," Ser. No. 07/998,179; "Apparatus and Method for aModified Half-Aperture Focus/Tracking/Data Sensor System," Ser. No.07/997,809; "Read/Write Laser-Detector-Grating Unit (LDGU) With AnOrthogonally-Arranged Focus and Tracking Sensor System," Ser. No.08/259,428; and "Multi-element Grating Beam Splitter With a ReflectionGrating Element For Use in Front Facet Subtraction," Ser. No.08/259,587, all assigned to the assignee of the present invention. Thedisclosures of these related Applications are incorporated herein byreference.

1. Field of the Invention

The present invention relates generally to optical heads used in opticalinformation storage and retrieval systems. More particularly, thepresent invention relates to focus and tracking sensor systems used inoptical heads to control the position of a radiation beam relative to anoptical storage medium.

2. Description of the Prior Art

FIG. 1 shows an exemplary optical information storage and retrievalsystem 10 in accordance with the prior art. An optical source 11, suchas a laser diode, provides a radiation beam which is collimated bycollimating lens 12. The collimated radiation beam is transmittedthrough a polarization beam splitter 13 and applied to a quarter-waveplate 14. The polarization beam splitter 13 transmits a linearpolarization of the radiation beam, such as a p-polarization, andquarter-wave plate 14 provides a circular polarization to the linearlypolarized radiation-beam. The circularly polarized radiation beam fromthe quarter-wave plate 14 is focused by objective lens 15 onto a datatrack 16B. The data track 16B is on a surface 16A of an optical storagemedium 16. The storage medium 16 may be, for example, any optical diskhaving a surface which interacts with an incident radiation beam.

The interaction with the storage medium surface 16A causes the incidentradiation beam to be reflected and diffracted therefrom. The resultingradiation beam, referred to herein as a return beam, is collimated byobjective lens 15 and then passes through the quarter-wave plate 14. Thequarter-wave plate 14 converts the circular polarized return beam to alinear s-polarized return beam. The linear s-polarization isperpendicular to the p-polarization of the radiation beam transmittedfrom polarization beam splitter 13. When the s-polarized return beam isapplied to the polarization beam splitter 13, it is reflected by asurface 13A of the beam splitter 13. The surface 13A may be, forexample, a multilayer coating which transmits p-polarized light andreflects s-polarized light. A focusing lens 17 converges the reflectedreturn beam onto a detector array 18. The detector array 18 detectsportions of the return beam which are used to provide, for example, afocus error signal (FES), a tracking error signal (TES), and a datasignal. The FES and TES are generally used to drive servo systems formaintaining the radiation beam in-focus and on-track, respectively,relative to the storage medium. The data signal is indicative of thedata stored on the data track scanned by the radiation beam. Thecomponents of an optical system which process, direct and detect thereturn beam to provide an FES and a TES, and in some cases also a datasignal, may be collectively referred to as a focus and tracking sensorsystem.

A number of other prior art optical systems split the return beam intotwo or more beams, each of which is directed to a separate detectorarray. For example, one detector array may be used to generate a TES anda data signal from one of the beams, while another detector is used togenerate an FES from another beam. Such an approach generally requiresadditional components and therefore increases the size, complexity andcost of the optical system.

Other prior art systems reduce the number of optical components bydirecting the return beam to an optical beam-separating device such as adual or four element prism followed by a single detector array. A fourelement prism will be referred to herein as a quad prism. The FES, TESand data signal are thus all collected using a single return path. Suchsystems will be referred to herein as single return path systems. Anexemplary single return path focus and tracking sensor system isdescribed in M. R. Latta et al., "Effect of Track Crossing on FocusServo Signals: Feedthrough," SPIE Proceedings, Vol. 1663, Optical DataStorage, 1992. The Latta et al. reference describes a system in which ananamorphic lens, or alternatively a combination of a spherical and acylindrical lens, is used to direct a single return beam to a quaddetector. The quad detector generates an astigmatic FES, a push-pullTES, and a data signal.

Another prior art single return path focus and tracking sensor system isdescribed in U.S. Pat. No. 4,712,205, issued to Smid et al. as assigneeof U.S. Philips Corporation. The Smid et al. system provides a dualhalf-aperture FES, a push-pull TES and a data signal using a singledetector array. The return beam is divided by a compound wedge into twoseparate beams which are directed to two dual element detectors in thesingle detector array. One significant problem with this system is highcross-talk between the FES and the TES. This and other problems with theSmid et al. system are described in more detail in the above-cited U.S.patent application Ser. No. 07/998,179.

FIG. 2 shows an exemplary implementation of another prior art singlereturn path focus and tracking sensor system 20, described in R.Katayama et al., "Multi-beam magneto-optical disk drive for parallelread/write operation," SPIE Proceedings, Vol. 1078, Optical Data StorageTopical Meeting, Jan. 17-19, 1989. The optical system 20 uses aradiation beam to read and/or write data on a data track 22 of anoptical storage medium 21. Only a portion of optical storage medium 21and data track 22 is shown in FIG. 2. The return beam reflected anddiffracted from the data track 22 is collimated by an objective lens 24and is incident on a quadrant prism 26. A quadrant prism is a type ofquad prism in which the four prism elements are arranged as quadrants.The dashed line 22A represents a tangent to data track 22 at a pointilluminated by the radiation beam. The dashed line 28 is substantiallyparallel to the data track 22 and represents a projection of the datatrack 22 on the quadrant prism 26. The dashed line 29 represents anoptical axis of the incident radiation beam. The quadrant prism 26includes four prism elements A, B, C and D. Each of the prism elementsA, B, C and D directs a portion of the return radiation beam fromobjective lens 24 through a focusing lens 30 to a detector array 32. Thedetector array 32 includes a number of detector elements a, b, c, c', dand d'. Prism elements A, B, C and D direct four different portions ofthe return beam toward detector elements a, b, c and c', and d and d',respectively. In the system 20, an FES, a TES and a data signal (DS) aregenerated from the portions of the return beam incident on the followingdetector elements:

FES=(c+d')-(c'+d)

TES=a-b

DS=a+b+c+c'+d+d'

The above relationships specify the manner in which signals from thevarious detector elements are combined to generate the FES, TES and datasignal, respectively. It should be noted that certain components presentin the system 10, such as the polarization beam splitter 13 and thequarter-wave plate 14, are not shown in system 20 for purposes ofclarity.

Depending upon the design of the above-described single return pathsystems, various amounts of optical cross-talk may originate from, forexample, diffracted radiation components and optical wavefrontaberrations in the return beam. The presence of optical cross-talk maylimit the effectiveness of certain optical systems, particularly thosesystems which utilize high performance servomechanisms to control focusand tracking of the incident radiation beam. Although the above-citedU.S. patent application Ser. No. 07/998,179 reduces the effect ofoptical cross-talk by implementing an orthogonality condition betweenthe focus and tracking sensors, it does so using separate optical pathsfor generating the focus and tracking signals. The need for additionalcomponents to create and process separate optical paths adverselyaffects the cost and complexity of the optical system.

As is apparent from the above, a need exists for optical systems whichgenerate focus and tracking error signals with low optical cross-talkwithout requiring separate optical paths and additional opticalcomponents.

SUMMARY OF THE INVENTION

The present invention provides an apparatus and method for generatingfocus and tracking error signals in a single return path optical system.In accordance with one aspect of the present invention, an apparatus isprovided which includes a multi-element beam separator having first,second, third and fourth separator elements for separating a return beamreflected and diffracted from a data track on a storage medium intofirst, second, third and fourth portions, respectively. In otherembodiments, the multi-element beam separator could include more or lessthan four elements, and more or less than four portions of the returnbeam could be used. A focusing lens may be used to converge the portionsof the return beam onto a detector array. The first and second separatorelements separate the return beam along at least one plane substantiallyparallel to a reference plane defined by an optical axis of theradiation beam and a tangent to the data track. The resulting first andsecond portions are detected in first and second detectors,respectively, and may be used to generate a tracking error signal. Thethird and fourth separator elements, arranged adjacent to and onopposite sides of the first and second separator elements, separate thereturn beam along planes substantially perpendicular to the referenceplane. The resulting third and fourth portions are detected in a thirdand fourth detector, respectively, and may be used to generate a focuserror signal. The apparatus may also be used to generate a data signalindicative of the data stored on the optical medium. The multi-elementbeam separator may be, for example, a quad prism having first, second,third and fourth prism elements suitably arranged to separate the returnbeam into the four portions.

In accordance with another aspect of the present invention, a method forproviding focus and tracking error signals for controlling anapplication of a radiation beam to a data track of an optical storagemedium is provided. The method includes the steps of separating a returnbeam resulting from application of the radiation beam to the data trackinto first, second, third and fourth portions. The first and secondportions are separated along at least one plane substantially parallelto a reference plane defined by an optical axis of the radiation beamand a tangent to the data track, and may be used to generate a trackingerror signal. The third and fourth portions are separated along planessubstantially perpendicular to the reference plane, and may be used togenerate a focus error signal. The method may also include the step ofconverging the first, second, third and fourth portions of the returnbeam on a detector array including first, second, third and fourthdetectors, respectively, which provide signals used to generate thetracking and focus error signals.

The present invention provides advantages in that the focus errorsignal, tracking error signal and data signal required in many opticalsystems may be generated with reduced optical cross-talk. The presentinvention generates these signals using a single return path without theneed for additional optical components, and therefore withoutsignificantly increasing the cost and complexity of the optical system.

Further features of the invention, its nature and various advantageswill become more apparent from the accompanying drawings and thefollowing detailed description of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exemplary optical information storage and retrieval systemin accordance with the prior art.

FIG. 2 is an exemplary single return path optical focus and trackingsensor system in accordance with the prior art.

FIG. 3 is an exemplary single return path optical focus and trackingsensor system in accordance with the present invention.

FIG. 4 is a more detailed view of an exemplary quad prism in accordancewith the present invention.

FIG. 5 is a cross-sectional view of the exemplary quad prism of FIG. 4taken along section line 5--5.

FIG. 6 is a cross-sectional view of the exemplary quad prism of FIG. 4taken along section line 6--6.

FIG. 7 is an alternative embodiment of a detector array suitable for usein the optical system of FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 3 shows an exemplary single return path focus and tracking sensorsystem 40 in accordance with the present invention. It should be notedthat certain optical system components, such as, for example, apolarization beam splitter and a quarter-wave plate, are not shown butmay also be included in the system 40. In addition, the system 40 isgenerally part of an optical system having a number of other components,such as, for example, an optical source and a collimating lens. Thearrangement of FIG. 3 may thus be implemented in any of a number ofdifferent optical systems and applications, including, for example, theoptical information and retrieval system 10 shown in FIG. 1. Amulti-element beam separator in accordance with the present inventioncould be located, for example, between polarization beam splitter 13 andfocusing lens 17, or between lens 17 and detector array 18, in theoptical system 10 of FIG. 1.

The exemplary system 40 of FIG. 3 processes a return beam reflected anddiffracted from a data track 42. The data track 42 is a diffractioncomponent-generating structure present on the surface of an opticalrecording medium. The structure diffracts the incident radiation beambecause the depth of the structure is generally a fraction of thewavelength of an incident radiation beam and introduces phasedifferences in the return beam. The term "data track" as used hereinincludes other diffraction component-generating structures such as, forexample, a raised structure or a groove in the Storage medium, a regionbetween two grooves in the storage medium, a series of unconnectedraised regions, and other optical path structures of appropriatedimension and refractive index such that diffraction patterns arecreated in response to an incident radiation beam.

The return beam is incident on an objective lens 44 which directs thereturn beam to a quad prism 46. The exemplary quad prism 46 includesfour prism elements E, F, G and H which may be constructed of, forexample, glass, plastic, molded glass or molded plastic. The prismelements E, F, G and H separate the return beam into four differentportions. In this exemplary embodiment, each of these portions isfocused on a different detector in a detector array 52. A focusing lens50 is arranged between the quad prism 46 and detector array 52 toconverge the separated portions of the return beam onto the appropriatedetectors in detector array 52. It should be understood that the quadprism 46 having prism elements E, F, G and H may be replaced with othertypes of multi-element beam separators having corresponding separatorelements capable of separating the return beam into a number of distinctportions.

It should be noted that data track 42 is only a portion of a longer datatrack which is typically arranged in a spiral configuration on thesurface of an optical storage medium such as a recordable compact disk(CD). The data track 42 is shown as a straight line in FIG. 3 becausethe radius of curvature for its corresponding spiral is generally muchlarger than a given portion of the data track around a point currentlyilluminated by the incident radiation beam. Such a portion may thereforebe considered substantially straight. A projection 48 of data track 42onto quad prism 46 may also be characterized as substantially parallelto a tangent 42A to the data track 42 at the point currently illuminatedby the incident radiation beam. A reference plane is defined by anoptical axis 49 of the incident radiation beam and the tangent 42A tothe data track 42. The data track 42, projection 48, and the tangent 42Aare each substantially perpendicular to the optical axis 49. Elements Eand F of quad prism 46 are thus arranged on opposite sides of thereference plane defined by optical axis 49 and tangent 42A. Elements Gand H are arranged adjacent to and on opposite sides of elements E andF, and are separated therefrom by planes which are substantiallyperpendicular to the reference plane.

The detector array 52 includes a first, a second, a third and a fourthdetector for detecting the first, second, third and fourth separatedportions of the return beam, respectively. The first and seconddetectors are single element detectors designated in FIG. 3 as detectorelements e and f. The third detector is a dual element detectorincluding detector elements g and g'. The fourth detector is a dualelement detector including detector elements h and h' Each detectorelement may be, for example, a photodiode, a group of photodiodes, oranother type of optical photodetector. It should be noted that thisparticular arrangement of detectors is exemplary only. The detectors mayinclude additional detector elements or fewer detector elements inalternative embodiments. In addition, each of the detectors need not bepart of a single detector array. As will be discussed in greater detailbelow, the prism elements and corresponding detector elements arearranged such that the optical cross-talk between focus and trackingsignals is minimized.

The return beam generally includes both a reflected component, alsoreferred to as a zeroth order diffracted component, and a number ofhigher order diffracted components diffracted from the optical storagemedium. A given diffraction order generally includes both a positive anda negative diffraction component. Although higher order diffractioncomponents may also be present, the present invention can be readilyunderstood without further consideration of diffraction componentsgreater than first order. When the reflected component overlaps with thefirst order diffracted components, interference occurs. Thisinterference may be directed to detectors e and f to provide, forexample, a push-pull TES. The two first order diffraction components maybe, for example, contiguous with an optical axis of the incidentradiation beam, and therefore both will overlap with the reflectedcomponent. It should be noted, however, that the present invention maybe utilized in systems in which the positive and negative diffractioncomponents overlap with each other as well as with the reflectedcomponent. Additional detail regarding diffraction components may befound in, for example, the above-cited U.S. patent application Ser. No.07/998,179, and in pp. 172-179 of A. Marchant, "Optical Recording: ATechnical Overview," Addison-Wesley, Reading, Mass., which areincorporated by reference herein.

FIG. 4 shows a portion of the exemplary quad prism 46 of the presentinvention in greater detail. In the exemplary embodiment shown, thefirst and second prism elements E and F are arranged on opposite sidesof a line 53 which is substantially parallel to data track 42, tangent42A thereto, and projection 48 thereof, and which lies in theabove-defined reference plane. The first and second prism elementsseparate the return beam into first and second portions, respectively,along at least one plane substantially parallel to the reference plane.The first and second prism elements could also be arranged on oppositesides of a plane parallel to the above-defined reference plane. Inaddition, the first and second prism elements could be arranged so as toseparate the return beam along a first and a second plane, respectively,both of which are substantially parallel to the reference plane. In suchan embodiment, the first and second prism elements are not contiguous,as in the embodiment of FIG. 4, but are instead separated by a gap. Anyportion of the return beam incident on a gap between the first andsecond prism elements could be suitably directed, in a well-knownmanner, onto one or more of the detectors in array 52. The third prismelement G is arranged adjacent to and on one side of the first andsecond prism elements E and F, on one side of a line 54 which issubstantially perpendicular to the data track 42, tangent 42A thereto,and projection 48 thereof. The line 54 lies in a plane substantiallyperpendicular to the above-defined reference plane. The fourth prismelement H is arranged adjacent to and on an opposite side of the firstand second prism elements E and F, on an opposite side of a line 56which is substantially perpendicular to the data track 42, tangent 42Athereto, and projection 48 thereof. The line 56 lies in another planesubstantially perpendicular to the above-defined reference plane. Thethird and fourth prism elements separate the return beam into third andfourth portions, respectively, along planes substantially perpendicularto the reference plane.

A tracking error signal (TES) may be generated from the first and secondportions of the return beam incident on the first and second detectors eand f of the detector array 52. The TES is generated in accordance withthe relationship e-f, which indicates that the signal generated bydetector element f is subtracted from the signal generated by detectorelement e to provide the TES. The first and second portions, as a resultof passing through the above-described quad prism 46, may each include adifferent diffraction component, such as either a positive or a negativediffraction component, of a given diffraction order, as well asundiffracted components. The diffracted components are those diffractedfrom the optical storage medium. It should be understood that, ingeneral, only part of any given diffraction component, rather than theentire component, falls within the objective lens aperture and willtherefore be incident on quad prism 46. References made herein to aparticular diffraction component are thus meant to include any part ofthat component.

A focus error signal (FES) may be generated from the third and fourthportions of the return beam incident on the third and fourth detectorsg, g' and h, h' of the detector array 52. An FES is generated inaccordance with the relationship (g+h')-(g'+h), which indicates that thesum of the signals generated by detector elements g' and h is subtractedfrom the sum of the signals generated by detector elements g and h' toprovide an FES. The third and fourth portions, as a result of passingthrough the above-described quad prism 46, may each include bothpositive and negative diffraction components of a given diffractionorder, as well as undiffracted components. The diffracted components arethose diffracted from the optical storage medium. Each of the detectorelements g, g', h and h' thus receives both diffraction components of agiven diffraction order. By subtracting the signals resulting fromdetection of the third and fourth portions of the return beam indetector elements g, g' and h, h', respectively, the diffractioncomponents of a given diffraction order substantially cancel out,thereby reducing optical cross-talk.

A data signal, indicative of the data stored on data track 42, may alsobe generated in the optical system 40. For example, a data signal couldbe generated by combining the signals generated by each detector elementin the detector array 52, in accordance with the relationshipe+f+g+g'+h+h'. Alternatively, signals from a subset of detector elementscould be combined to generate a data signal.

System 40 may also include electronic circuitry (not shown) forcombining signals generated by the detector elements of array 52. Theelectronic circuitry may include adders, subtracters or other types ofsignal combiners for generating focus error, tracking error, and datasignals in accordance with the above-described relationships. Suchelectronic circuitry is generally well-known in the art and willtherefore not be further described herein.

As noted above, the arrangement of prism elements shown in FIGS. 3 and 4is exemplary only, and alternative embodiments of the present inventionmay utilize other arrangements. For example, the various elements of theprism 46 may be separated by lines which deviate from the parallel orperpendicular lines shown in FIG. 4 by up to about ten percent. Theterms "substantially parallel" and "substantially perpendicular", asused herein, include deviations of at least ten percent from paralleland perpendicular, respectively. Although the amount of opticalcross-talk may increase as a result of such deviations, an improvementover most current prior art systems would generally still be obtained.In addition, although the separator elements E, F, G and H are generallyshown in FIGS. 3 and 4 as contiguous portions of a circularmulti-element prism, in other embodiments these elements could bearranged in a variety of other ways. For example, contiguous elementscould be separated and any resulting gaps filled with an opaquematerial, although part of the return beam may be lost in such anarrangement. Furthermore, although a four element beam separator ispreferred in many applications, a multi-element beam separator inaccordance with the present invention may include either more or lessthan four separator elements. For example, in an embodiment which doesnot use a data signal, an alternative multi-element beam separator couldinclude only the elements E, F and G, or E, F and H. In an embodimentwhich uses a data signal, the elements E and F could be modified in sizeand shape to also receive the portion of the return beam which wouldotherwise fall on the removed element, such that the entire return beamis still incident on the detector array. A beam separator withadditional elements could be suitably arranged to separate the returnbeam into portions which, when detected, generate signals which may becombined in accordance with the present invention such that opticalcross-talk is minimized. One skilled in the art could readily modify themulti-element beam separator 46, detector array 52, and theabove-described relationships for generating an FES, a TES and a datasignal, to accommodate such alternative embodiments.

FIG. 5 shows a cross-sectional view of the exemplary quad prism 46 takenalong the section line 5--5 of FIG. 4. This cross-sectional view showsthat the fourth prism element H may include an outer surface sloping inone direction. The outer surface of a given prism element refers to asurface on which the return beam is incident before it is separated bythe prism. An inner surface refers to a surface exited by a separatedportion of the return beam. The third prism element G may include anouter surface sloping in a direction opposite that of the outer surfaceof fourth prism element H. These sloping surfaces allow the quad prism46 to direct the third and fourth portions of the return beam toward thedetectors g, g' and h, h' on detector array 52.

FIG. 6 shows a cross-sectional view of the exemplary prism 46 takenalong the section line 6--6 of FIG. 4. This cross-sectional view showsthat the second prism element F may include an outer surface slopingprimarily in one direction. The first prism element E may include anouter surface sloping primarily in the same direction as the outersurface of second prism element F. The outer surfaces of first andsecond prism elements E and F also slope in other directions. Thesesloping surfaces allow the prism 46 to direct the first and secondportions of the return beam toward the detectors e and f on detectorarray 52. It should be understood that the prism element surfaces shownin FIGS. 5 and 6 are exemplary only, and are arranged as shown in orderto direct the separated portions of the return beam to an appropriatedetector in detector array 52. In other embodiments these surfaces maybe modified to provide any of a number of alternative arrangementswithout deviating from the teachings of the present invention. Forexample, the direction, amount and type of slope may be varied, or aninner surface rather than an outer surface of a given prism elementcould be shaped so as to suitably direct the return beam. One skilled inthe art could readily determine appropriate dimensions, refractiveindices, and other parameters for any number of different prismelements, or more generally beam separator elements, suitable for use inthe present invention.

FIG. 7 shows an alternative detector array 72 suitable for use with thepresent invention. Each of the portions of the return beam separated bythe prism elements of the prism 46 are focused onto a different detectorof the detector array 72 using focusing lens 50 or another suitablefocusing device. Exemplary focus spots 74, 76, 78 and 80 indicate anarea of each detector on which the first, second, third and fourthreturn beam portions, respectively, may be focused when the incidentradiation beam is on-track and in-focus relative to the optical storagemedium. The detector array 72 includes first, second, third and fourthdetectors for detecting the first, second, third and fourth portions ofthe return beam, respectively. The first and second detectors includesingle detector elements e and f, respectively. The third and fourthdetectors are dual element detectors including detector element pairs g,g' and h, h', respectively. In this embodiment, as in the detector array52 of FIG. 3, the division between the detector element pairs g, g' andh, h' is along a line substantially perpendicular to the reference planedefined by optical axis 49 and tangent 42A to data track 42. The firstand second detector elements e and f are arranged on opposite sides ofthe detector array 72, rather than on the same side as in detector array52 of FIG. 3. The sloping surfaces of prism elements E and F could bereadily modified in a well-known manner to direct the first and secondportions of the return beam to detectors e and f, respectively, of array72.

In general, the orientation and location of the detector elements e andf is not critical to the operation of the present invention, and thearrangements in FIGS. 3 and 7 or other alternative arrangements may bechosen in order to satisfy detector array packaging constraints or othercriteria. The position of the third and fourth detectors g, g' and h, h'may also be varied but the division between the pairs should generallybe oriented in a direction substantially perpendicular to the referenceplane.

Although the foregoing detailed description has described the presentinvention primarily in terms of an illustrative optical informationstorage and retrieval system, it should be understood that theembodiments described are exemplary only. Many variations may be made inthe arrangements shown, including, for example, the type of opticaldevice used to separate the return beam and the arrangement, shape andnumber of separating elements, the number of portions into which thereturn beam is separated, the arrangement of detectors and detectorelements onto which the portions of the return beam are focused, and thetype and arrangement of optical components for directing the incidentand return radiation beams in the optical system. These and otheralternatives and variations will be readily apparent to those skilled inthe art, and the present invention is therefore limited only by theappended claims.

    ______________________________________                                        PART LIST                                                                     ______________________________________                                        A, B, C, D, E, prism elements                                                 F, G, H                                                                       a, b, c, c', d,                                                                              detectors                                                      d', e, f, g, g',                                                              h, h'                                                                         10             optical system                                                 11             optical source                                                 12             collimating lens                                               13             polarization beam splitter                                     13A            surface                                                        14             quarter-wave plate                                             15             objective lens                                                 16             optical storage medium                                         16A            optical storage medium surface                                 16B            data track                                                     17             focusing lens                                                  18             detector array                                                 20             focus and tracking sensor system                               21             optical storage medium                                         22             data track                                                     22A            tangent to data track                                          24             objective lens                                                 26             quadrant prism                                                 28             projection                                                     29             optical axis                                                   30             focusing lens                                                  32             detector array                                                 40             focus and tracking sensor system                               41             optical storage medium                                         42             data track                                                     42A            tangent to data track                                          44             objective lens                                                 46             quad prism                                                     48             projection                                                     49             optical axis                                                   50             focusing lens                                                  52             detector array                                                 53, 54, 56     lines                                                          74, 76, 78, 80 focus spots                                                    ______________________________________                                    

What is claimed is:
 1. An apparatus for providing tracking and focuserror signals for controlling an application of a radiation beam to adata track of an optical storage medium, said apparatus comprising:amulti-element prism having at least a first separator element, a secondseparator element and a third separator element, with at least one ofthe separator elements having a linearly-sloped beam-directing surface,the multi-element prism adapted to separate a return beam resulting fromsaid application of said radiation beam to said data track into at leasta first portion, a second portion and a third portion, respectively,such that said first and said second separator elements separate saidreturn beam along at least one plane substantially parallel to areference plane defined by an optical axis of said radiation beam and atangent to said data track, and said third separator element, arrangedadjacent to and on one side of the first and second separator elements,separates said return beam along a plane substantially perpendicular tosaid reference plane; a first detector to detect said first portion ofsaid return beam; a second detector to detect said second portion ofsaid return beam; and a third detector to detect said third portion ofsaid return beam.
 2. The apparatus of claim 1 further including:a fourthseparator element in said multi-element prism arranged adjacent to saidfirst and second separator elements on a side opposite said thirdseparator element, and having a linearly-sloped beam-directing surfaceto separate a fourth portion of said return beam along another planesubstantially perpendicular to said reference plane; and a fourthdetector to detect said fourth portion of said return beam.
 3. Theapparatus of claim 2 wherein said third and said fourth detectors areused to generate said focus error signal from said third and said fourthportions of said return beam.
 4. The apparatus of claim 3 wherein saidthird detector includes detector elements g and g', said fourth detectorincludes detector elements h and h', and said focus error signal isgenerated in accordance with the relationship (g+h')-(g'+h).
 5. Theapparatus of claim 2 wherein said third and said fourth portions of saidreturn beam each include both a positive and a negative diffractioncomponent of a given diffraction order.
 6. The apparatus of claim 2wherein said first detector includes a detector element e, said seconddetector includes a detector element f, said third detector includesdetector elements g and g', and said fourth detector includes detectorelements h and h', and said apparatus further provides a data signalindicative of data stored on said data track in accordance with therelationship e+f+g+g'+h+h'.
 7. The apparatus of claim 2 wherein saidthird and said fourth detectors each include two detector elementsdivided along a line perpendicular to said reference plane.
 8. Theapparatus of claim 1 wherein said first and said second portions of saidreturn beam each include a different diffraction component of the samediffraction order.
 9. The apparatus of claim 1 wherein said first andsaid second detectors are used to generate said tracking error signalfrom said first and second portions of said return beam.
 10. Theapparatus of claim 9 wherein said first detector includes a detectorelement e and said second detector includes a detector element f andsaid tracking error signal is generating in accordance with therelationship e-f.
 11. The apparatus of claim 1 further including:adetector array including said first detector, said second detector andsaid third detector; and a focusing lens arranged between saidmulti-element prism and said detector array to converge said portions ofsaid return beam on said detectors.
 12. A method for providing trackingand focus error signals for controlling an application of a radiationbeam to a data track of an optical storage medium, said methodcomprising the steps of:separating a return beam resulting from saidapplication of said radiation beam to said data track into at least afirst portion, a second portion and a third portion using first, secondand third separator elements, respectively, of a multi-element prism,wherein at least one of the separator elements has a linearly-slopedbeam-directing surface, such that said first and said second portionsare separated along at least one plane substantially parallel to areference plane defined by an optical axis of said radiation beam and atangent to said data track, and said third portion is separated along aplane substantially perpendicular to said reference plane; and detectingsaid first portion, said second portion and said third portion of saidreturn beam in a first detector, a second detector and a third detector,respectively.
 13. The method of claim 12 wherein said step of separatingsaid return beam further includes separating a fourth portion of saidreturn beam along another plane substantially parallel to said referenceplane, using a fourth separator element in said multi-element prism, andsaid step of detecting further includes detecting said fourth portion ina fourth detector.
 14. The method of claim 13 further including thesteps of:generating said tracking error signal from said first andsecond portions of said return beam detected in said first and saidsecond detectors, respectively; and generating said focus error signalfrom said third and said fourth portions of said return beam detected insaid third and said fourth detectors, respectively.
 15. The method ofclaim 14 wherein said step of detecting said first and said secondportions of said return beam in said first and said second detectors,respectively, further includes detecting said first portion in adetector element e, and said second portion in a detector element f, andwherein said step of generating said tracking error signal furtherincludes the step of generating said tracking error signal in accordancewith the relationship e-f.
 16. The method of claim 14 wherein said stepof detecting said third and said fourth portions of said return beam insaid third and said fourth detectors, respectively, further includesdetecting said third portion in a dual element detector having detectorelements g and g' and said fourth portion in a dual element detectorhaving detector elements h and h', and wherein said step of generatingsaid focus error signal includes generating said focus error signal inaccordance with the relationship (g+h')-(g'+h).
 17. The method of claim12 further including the step of converging said portions of said returnbeam on a detector array including said first detector, said seconddetector and said third detector.
 18. A multi-element prism forseparating a return beam, resulting from application of a radiation beamto a data track of an optical storage medium, into a number of portions,said prism comprising:a first separator element to separate a firstportion of said return beam, said first separator element arranged onone side of a plane substantially parallel to a reference plane definedby an optical axis of said radiation beam and a tangent to said datatrack; a second separator element to separate a second portion of saidreturn beam, said second separator element arranged on an opposite sideof said plane substantially parallel to said reference plane, said firstand said second separator elements separating the return beam along saidplane substantially parallel to the reference plane; a third separatorelement to separate a third portion of said return beam; and a fourthseparator element to separate a fourth portion of said return beam, saidthird and said fourth separator elements arranged adjacent to and onopposite sides of said first and said second separator elements, andseparating the return beam along planes substantially perpendicular tosaid reference plane; wherein at least one of said first, second, thirdand fourth separator elements has a linearly-sloped beam-directingsurface.